It is well known to provide tanks, generally referred to as fish tanks, having transparent walls so that fish, swimming in water in the tanks, may be observed from the exterior of the tank. The fish may be bred, grown or maintained as a hobby, or tanks containing the fish may be displayed for their decorative or aesthetic effect to enhance the ambience of a home, office or place of public accommodation such as a hotel, motel, etc. The fish require oxygen for survival so an air pump is provided for supplying fresh air to the water.
FIGS. 11 and 12 show one example of an air pump P presently used to feed air into the water contained in a fish tank. The pump casing 1A comprises a plastic main casing body 1, open at the bottom side as viewed in FIG. 12, and a cover 6 detachably fitted to the casing body 1 so as to close the bottom side. Disposed within the casing body are an electromagnet 2, a vibrator in the form of an arm 4 mounted on a support 10 and having a permanent magnet 3 fixed to one end, and an elastic diaphragm member 5a made of rubber and interfitted with a second member 5d to form a pump chamber 5e. Member 5d is provided with an inlet valve 5b connected to an air suction opening 8 and a discharge valve 5c connected to an air discharge opening 9. The vibrator arm 4 is fastened to the diaphragm member 5a.
When an alternating voltage is applied through the power supply cord 11, the electromagnet 2 is actuated and the vibrator arm 4 vibrates, as indicated by the double-headed arrow in FIG. 12, as a result of the electromagnetic forces acting between the electromagnet 2 and the permanent magnet 3. As arm 4 vibrates, it moves the diaphragm member 5a fastened thereto so that the volume of the pump chamber 5e alternately increases and decreases and air in the chamber is alternately expanded and compressed. During the time of the expansion, the inlet valve 5b is opened and the discharge valve 5c is closed so that air A is sucked into the diaphragm chamber 5e from the interior of the casing. An air inlet 7 in the casing cover 6 permits entry of replacement air into the casing. During the time of the compression, the inlet valve 5b is closed and the discharge valve 5c is opened so that air in the diaphragm chamber 5a is discharged through the air discharge opening 9 to the outside of the casing. A flexible hose 12 conveys the discharged air to the water in a tank.
An air pump having the arrangement shown in FIGS. 11 and 12 is simple in structure, can be produced at a low price and exhibits a stable operation so that the rate of air discharge (volume per unit of time) is substantially constant. However, the air pump does have disadvantages, the most serious being that it generates considerable audible noise. When the background noise level in the surrounding environment is comparatively high, as in the lobby of a hotel in the daytime for example, the noise of the pump is not a serious problem. On the other hand, when the background noise level is low, as might be the case at night in sleeping quarters for example, the noise generated by operation of the pump becomes more noticeable. The pump thus generates noise which cannot be ignored, is irritating to those who hear it, and disturbs sleep or concentration at work or study.
Workers in the art have attempted to reduce the pump noise by various methods including (1) improving air-tightness of the casing, (2) mounting the electromagnet 2 and the components forming pump 5 to the main casing body 1 through a vibration preventing material, or (3) by a combination of improved air-tightness and improved mounting of the components. The first method, that is, improvement of the air-tightness of the casing, is the most frequently used. In this case through-holes in the casing body 1 and the cover 6, such as the through-hole for electrical leads 11, and the narrow spaces at the joint between the casing body 1 and cover 6 are filled with a sealing material such as a silicone resin. The air inlet 7, which permits entry of replacement air, can not be sealed so it is filled with a filter material 7a which may be glass fiber or similar material.
The above sealing arrangement considerably reduces the noise emitted into the surrounding environment by the pump P. FIGS. 13 and 14 are plots of measured acoustic pressure level in decibels versus frequency for a conventional unsealed air pump P (FIG. 13), and for the same pump sealed with a silicone resin sealing material as described above (FIG. 14). In both cases the measurements were made in an otherwise noiseless room with a silencer installed on the outlet of the pump.
Comparing FIGS. 13 and 14, it is seen that in the case of the sealed air pump P, the noise is reduced slightly at 60 Hz and greatly in a frequency range above 200 Hz. On the other hand, there is only a slight reduction of noise in the low frequency range between about 60 Hz to 200 Hz. Thus, noise abatement by the sealing arrangement does not completely solve the noise problem. In situations where the background noise level is low, such as at night in a comparatively quiet neighborhood, the noise produced by the pump is sufficiently loud to be a source of annoyance and irritation.
In the second method of reducing pump noise, the pump 5 and the electromagnet 2 are fixed to the casing body 1 through vibration-proof rubber, etc., and the cover 6, which serves as the base of the air pump, is made of rubber so that a vibration absorbing arrangement is obtained and the noise is reduced. This method reduces noise propagated through solid parts of the pump but does little to suppress noise propagated by air. When the noise of the vibration-proof type air pump P is actually measured, it is found that the noise reducing effect is similar to that of the sealed air pump. Therefore, the noise cannot be reduced to a satisfactory degree using vibration suppression in spite the large number of parts employed and the high manufacturing costs involved.
Further, the inventor of the present invention has found that in an air pump as shown in FIGS. 11 and 12, vibrations of the electromagnet 2, vibrator arm 4 and diaphragm 5a, do not comprise the only sources of noise. Operations of the inlet valve 5b and the discharge valve 5c also generate noise. Based on this finding he has previously invented a low-noise air pump having an arrangement such that a synthetic resin pipe about 30 to 50 cm long is provided in the casing body 1, and air introduced into the casing through the pipe is sucked into the diaphragm chamber 5e of the pump.
It was found that the above method using the synthetic resin hose can reduce the noise of the air pump P greatly and produce an excellent practical effect. However, in order to provide the synthetic resin hose in the casing body 1, a winding bobbin and a holding member for holding the bobbin are additionally required. Contact of the synthetic resin hose with the casing body 1 becomes a new source of noise. For this reason, the bobbin around which the synthetic resin hose is wound should be installed accurately, and thus this method has a disadvantage in that it requires great care during assembly of the pump.
The hose reduces noise at the inlet side of pump P. Since the noise at the discharge side of the pump P should be also be reduced, as by using an expansion-type silencer, accessories for the pump increase its cost.